Macrocephaly, Increased Intracranial Pressure, and ...

[Pages:10]ORIGINAL ARTICLE

Macrocephaly, Increased Intracranial Pressure, and Hydrocephalus in the Infant and Young Child

Alexandra T. Vertinsky, MD and Patrick D. Barnes, MD

Abstract: Macrocephaly, increased intracranial pressure, and hydrocephalus are common related conditions that lead to crosssectional imaging of the infant and young child. Imaging plays a central role in establishing the diagnosis and guiding disposition and treatment of these patients. In this review, a general overview is provided, and the more common causes of hydrocephalus are presented, including posthemorrhage, postinfection, developmental malformations, and masses. Imaging guidelines are also outlined for initial evaluation and follow-up, along with a discussion of the imaging features of shunt malfunction.

Key Words: macrocephaly, hydrocephalus, pediatric, MRI

(Top Magn Reson Imaging 2007;18:31Y51)

M acrocephaly (MC), increased intracranial pressure (ICP), and hydrocephalus (HC) are common related conditions that lead to cross-sectional imaging of the infant and young child. Imaging plays a central role in establishing the diagnosis and guiding disposition and treatment of these patients. In this review, a general overview is provided, and the more common causes of HC are presented, including posthemorrhage, postinfection, developmental malformations, and masses (Tables 1, 2). Imaging guidelines are also outlined for initial evaluation and follow-up, along with a discussion of the imaging features of shunt malfunction.

MACROCEPHALY Macrocephaly is defined as a head circumference more than 2 SD above the mean. Common causes of MC include familial megalencephaly (larger-than-normal brain mass), benign extracerebral collections of infancy (BECC) and HC. Macrocephaly without HC may also be seen in some genetic, metabolic, and dysplastic syndromes, or may be caused by tumors and cysts, pseudotumor cerebri, or subdural collections (eg, hematomas, hygromas).1 Evaluation of head growth rate (ie, serial head circumferences) along with assessment of developmental milestones, perinatal history, and signs of ICP is important for differential diagnosis, urgency of imaging, and radiological interpretation.2 Macrocephaly with normal growth rate and normal neurological examination is reassuring and is characteristic of benign megalencephaly, which is usually familial. Dysplastic

From the Stanford University Medical Center, Stanford, CA. Reprints: Patrick D. Barnes, MD, Departments of Radiology, Pediatric MRI

and CT, Room 0511, Lucille Packard Children_s Hospital, 725 Welch Road, Palo Alto, CA 94304 (e-mail: pbarnes@stanford.edu). Copyright * 2007 by Lippincott Williams & Wilkins

megalencephaly is often associated with developmental delay, seizures, a neurocutaneous syndrome (eg, neurofibromatosis), a genetic syndrome (eg, Soto syndrome), hemimegalencephaly (Fig. 1), or elevated venous pressure (eg, achondroplasia) (Fig. 2). Macrocephaly from Brebound[ or Bcatch-up[ brain growth occurs in the thriving infant after prematurity or after a period of deprivation or serious illness. Familial, dysplastic, and rebound types of MC may manifest mild to moderate degrees of ventricular or subarachnoid space dilatation.3Y5

MC and accelerated head growth without elevated pressure and with normal neurological exam may occur as nonprogressive subarachnoid space dilatation with or without ventricular enlargement. This pattern is most commonly referred to as BECC, but has also been termed as Bbenign enlargement of the subarachnoid spaces,[ Bbenign infantile HC,[ and Bbenign external HC.[1,3Y7 The cause is unknown, but it may be related to delayed development of parasagittal dural channels responsible for cerebrospinal fluid (CSF) resorption in young children (who have few arachnoid villi). Accelerated head growth may continue until 12 to 18 months of age and then usually stabilizes as a form of megalencephaly. Imaging features of BECC include normal to mildly enlarged lateral and third ventricles and symmetric enlargement of the frontal subarachnoid spaces, interhemispheric fissure, and Sylvian fissures (Fig. 3).4 These extracerebral collections must be differentiated from subdural collections. On magnetic resonance imaging (MRI), the visualization of 2 layers of differing signal intensity or of abnormal signal intensity related to blood products, rather than CSF, is helpful to identify subdural collections (Fig. 4).5 The presence of bridging cortical draining veins extending through an extraaxial collection is supportive of, but not specific for, the subarachnoid space.7 Infants with BECC may be at increased risk of subdural hematoma, spontaneously or from minor trauma, resulting from the stretching of cortical veins.6,8

MC with accelerated head growth due to progressive HC is usually associated with signs of ICP and often with declining milestones. The exception may be an infant or child with preexisting brain injury such as the premature infant with HC from intraventricular hemorrhage and coexistent periventricular leukomalacia (PVL). Other causes of MC with megalencephaly, hydrocephaly, or craniomegaly (enlarged calvarium) include lipid storage disease, leukodystrophies, cranial dysplasias, and marrow hyperplasia secondary to chronic hemolytic anemia.

INCREASED ICP Symptoms and signs of ICP, depending on age, include MC, accelerating head circumference, full or bulging

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Vertinsky and Barnes TABLE 1. Overview of Clinical Presentations

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fontanelle, split sutures, poor feeding, vomiting, irritability, vision impairment, headache, lethargy, stupor, encephalopathy, Parinaud syndrome, sixth nerve palsy, hypertonia with hyperreflexia, and papilledema.9,10 The causes of ICP include trauma, hemorrhage, acute hypoxic-ischemic insult, infection, parainfectious sequela, metabolic derangement, HC, tumors, pseudotumor cerebri, and universal craniosynostosis.9Y16 Imaging is indicated to define a mass, fluid collection, edema, or HC. The mass may be a cyst, neoplasm, abscess, or hematoma. The abnormal fluid collection may be subdural or epidural, whether a hematoma, empyema, effusion, or hygroma. Edema may be traumatic (Fig. 5), hypoxic-ischemic (Fig. 6), toxic (eg, lead poisoning), metabolic (eg, ketoacidosis), infectious (meningitis or encephalitis), parainfectious (acute disseminated encephalomyelitis, Reye syndrome), or due to pseudotumor.

HYDROCEPHALUS A common cause of MC and ICP in childhood is HC. Hydrocephalus is the state of excessive CSF volume with progressive enlargement of the ventricles, subarachnoid spaces, or both.17Y20 Hydrocephalus may be caused by an imbalance between CSF production and absorption, by a blockage of CSF flow, or from alterations in ventricular compliance and CSF pulse pressure.17,20 Hydrocephalus due

TABLE 2. Macrocephaly, ICP, and HC

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FIGURE 1. Hemimegalencephaly with macrocephaly and epilepsy. Prenatal US showed ventriculomegaly. Axial noncontrast CT at 2 days of life shows enlarged right cerebral hemisphere with a large dysplastic right lateral ventricle and thickened cortex (A). Axial T2 images (B, C) and coronal short T inversion recovery (D) demonstrate enlargement of the right hemisphere and lateral ventricle, prominent trigone (white arrow) and occipital horn, and thickened gyri.

to CSF overproduction is very rare but may occur with choroid plexus papilloma (CPP) or villus hypertrophy. Hydrocephalus due to CSF flow block or absorptive block may be described as Bcommunicating[ when the block occurs outside the ventricular system (eg, basal cisterns or parasagittal arachnoid villi) and Bnoncommunicating[ when there is intraventricular obstruction (at or proximal to the fourth ventricular outlets).9,18

Most childhood HC occurs in infancy (Table 3). The most common cause is acquired adhesive ependymitis or arachnoiditis after hemorrhage or infection.9,17,18 Hydrocephalus is a well-known sequela of neonatal intracranial hemorrhage especially in the preterm (PT).16 Prenatal or postnatal infection may also lead to HC.11,12 By far the most common developmental cause of HC is the Chiari II malformation associated with myelocele/myelomeningocele (MMC). The HC often develops, or progresses, after repair of the spinal defect.9,21 Other common developmental causes include aqueductal anomalies (forking, stenosis, septation, gliosis) and the Dandy-Walker-Blake spectrum of retrocerebellar cysts.22 Less common or rare causes of HC include

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Macrocephaly, ICP, and HC

FIGURE 2. Achondroplasia. Axial T2 image shows mild prominence of the lateral ventricles and enlarged subarachnoid spaces (A). Sagittal T2 (B) image show a small skull base and small foramen magnum (white arrow).

foramen of Monro atresia, skull base anomalies, intracranial cyst, craniosynostosis, encephalocele (Fig. 7), holoprosencephaly (Fig. 8), hydranencephaly, and lissencephaly (Fig. 9).23

IMAGING EVALUATION The imaging diagnosis of HC may be made with ultrasonography (US), computed tomography (CT), or MRI. Although ventricular enlargement in the absence of atrophy or underdevelopment suggests HC, this finding alone may not be specific. Clinical features of ICP or progressive head enlargement supports the diagnosis of HC. Additional imaging features supportive of HC include ballooned enlargement of the anterior and posterior recesses of the third ventricle, rounded configuration of the lateral ventricles with decreased ventricular angles, accentuated CSF flow voids on MRI, and dilatation of the temporal horns proportionate with that of the lateral ventricle bodies.6,9,24Y26 Disproportionate enlargement of ventricles relative to sulci is not as reliable in differentiating underdevelopment or atrophy

from HC in infants and young children. Periventricular edema due to transependymal CSF flow or hydrostatic stasis from elevated intraventricular pressure may be evident as blurred or ill-defined ventricular margins. This finding favors acute/ subacute or progressive HC (Figs. 10, 11). However, the normally high water content of the immature white matter may obscure edema due to HC in the infant.6,9,24Y26

Hydrocephalus in the fetus and infant is often diagnosed with US or CT initially.9,27 Doppler US using the graded fontanelle compression technique may be used to identify infants with ventriculomegaly and ICP, and help determine the need and timing for shunting.28 Magnetic resonance imaging may be indicated to further delineate HC when surgery is more specifically directed beyond that of simple shunting, as may occur in the setting of a retrocerebellar cyst, isolation of the fourth ventricle, porencephaly, postventriculitis encystment, or a ventricular tumor.9,25,26 Proper catheter placement for management of HC related to a cyst in the Dandy-Walker-Blake spectrum (ie, shunting of the cyst, the ventricles, or both) often depends upon patency of the aqueduct. Upward or downward herniation may occur due to unbalanced decompression of the ventricles relative to the cyst.9,22 Magnetic resonance imaging has an important role in planning the surgical management of these cases.

Endoscopic third ventriculostomy (ETV) is a relatively new neurosurgical procedure. It is often used for patients with obstruction at or distal to the posterior third ventricle who have patent subarachnoid spaces. The obstruction is bypassed by a surgical opening made in the floor. Endoscopic third ventriculostomy has been used in children primarily for decompression of HC due to aqueductal stenosis in the absence of communicating HC or immaturity of the arachnoid villi.17,29,30 More recently, it has undergone evaluation for treating other etiologies of HC.31Y37 Although ETV is still considered to be more effective in patients older than 2 years, it is being advocated in younger patients. The success rate is enhanced in infants with a defined anatomic obstruction. However, moderate success (?40%Y60%) has

FIGURE 3. Benign extracerebral collections of infancy. A, Noncontrast computed tomographic images demonstrate normal brain densities with prominent frontal subarachnoid spaces bilaterally and slight widening of the ventricles and sylvian fissures. BYD, Magnetic resonance imaging in another infant. Axial T2 (B) and FLAIR (C) plus coronal short T inversion recovery (D) images demonstrate linear cortical veins (arrows) traversing the enlarged extracerebral spaces that conform to CSF intensities on all sequences.

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FIGURE 4. Subdural hematomas in a 5-month-old boy with macrocephaly and seizures. Magnetic resonance imaging demonstrates loculated subdural hematomas with blood products of differing ages. A, Sagittal T1 image shows mixed-intensity subdural collections consistent with acute (or possibly hyperacute) hemorrhage. Note the line separating the collection from the low-intensity subarachnoid space (white arrow). B, Axial FLAIR image shows the separation of the collection from the subarachnoid space (especially over the left convexity, white arrow). The collections have mixed high intensity with fluid-fluid levels (black arrow) and septations present. Cortical veins are not seen crossing the collections. There is mass effect with right to left shift. C, Axial gradient echo (GRE) image shows susceptibility with hypointensities along the septations (white arrow). D, Coronal postgadolinium T1 image shows enhancement of the cortical vessels, and margins of the collections, but no traversing veins.

been documented for infants with posthemorrhagic or postinfectious HC, as well as for HC associated with MMC and Chiari II.31 Magnetic resonance imaging facilitates surgical planning by delineating the anatomy of the third ventricle, the prepontine cistern, and the course of the basilar artery. It also assists in the assessment of ETV patency after surgery.38

Although CT may be a practical screening examination for HC after infancy, MRI is preferred because neoplasm becomes the leading consideration.9 The superior contrast resolution, multiplanar imaging capability, and ability to assess parameters such as flow and tissue anisotropy make MR I the procedure of choice for delineation of anatomy and extent of tumor for planning of surgery, radiotherapy, and

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chemotherapy, as well as for follow-up of tumor response and treatment effects. Magnetic resonance imaging may offer more supportive or causative information when the CT or US demonstrate nonspecific ventriculomegaly. Unexplained HC requires a thorough investigation for an occult inflammatory or neoplastic process.9 Magnetic resonance imaging may demonstrate a periaqueductal tumor in Bpresumed[ aqueductal stenosis or leptomeningeal enhancement in inflammatory or neoplastic infiltration (eg, due to granulomatous infection or tumor seeding). Magnetic resonance imaging may also clarify nonspecific extracerebral collections first identified on CT or US by differentiating benign infantile extracerebral collections from subdural hematomas as discussed earlier.4,5,9,14

Follow-Up Evaluation Treatment of HC involves the resection or decompres-

sion of the causative mass, ventricular diversion, or both.9,13,19,30 Prognosis depends on the origin of the HC and on timing of the treatment. The prognosis for HC associated with an extensive brain malformation or diffuse brain injury is poor. The secondary effects of HC on the malformed, injured, or developing brain may be devastating. Unchecked progressive HC produces interstitial edema, ependymal disruption, spontaneous ventriculostomy, possible herniation, and subependymal gliosis, demyelination, cystic leukomalacia, neuronal injury, and atrophy.9,17,29,30 The goal of shunting is to reduce pressure to safe levels and to protect brain tissue. Successful shunting is demonstrated on followup imaging as a proportionate decrease in ventricular size and reestablishment of brain mantle thickness (Fig. 12).9,29,30

The follow-up imaging of shunted (eg, ventriculoperitoneal [VP]), nonneoplastic HC may be adequately done with US or CT.27 Ultrasonography is also an ideal guide for shunt placement intraoperatively and for shunt placement evaluation on follow-up. After loss of the acoustic window in older infants and children, CT becomes the procedure of choice for routine follow-up and for evaluating shunt complications, including malfunction and subdural fluid collections.9 Magnetic resonance imaging provides multiplanar anatomical and multiparametric delineation, including CSF flow dynamics, which can be helpful for assessment of complex, compartmentalized, or encysted HC.9,20,25,26 In general, after ETV, ventricular size decreases more slowly and to a lesser degree than seen after shunt placement.38Y40 Visualization of flow through the third ventriculostomy by MRI along with demonstration of a moderate decrease in ventricular size correlates with ETV success.38,39 Ventriculostomy patency is assessed using a thin-slice sagittal T2 fast spin echo sequence. Visualization of a CSF flow-void in the third ventricular floor extending to the suprasellar cistern is an indicator of shunt patency (Fig. 13).41

SPECIFIC CAUSES OF HC IN CHILDHOOD

Posthemorrhagic HC and Venous Hypertension Germinal matrix (GM)/intraventricular hemorrhage

(IVH), which occurs in 20% of infants born before 34 weeks'

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Macrocephaly, ICP, and HC

FIGURE 5. Traumatic injury in a 2-year-old boy struck by a car. A, Noncontrast CT shows left depressed skull fractures (white arrow), frontal and temporal cerebral edema with loss of gray-white matter differentiation, hemorrhages, and mass effect (posttraumatic infarction or contusion). A left hemicraniectomy was done for impending herniation. Postcraniectomy axial T2 (B) and GRE (C) images show increased hemorrhage and gyral swelling mainly in the left MCA distribution with transcraniectomy herniation. This is more consistent with infarction than with contusion.

gestation, is one of the most common severe complications among PT and low-birth-weight infants and often results in HC.42 Germinal matrix/IVH is the sequela of a combination of intravascular, vascular, and extravascular factors that are related to prenatal, natal, and postnatal events. Intravascular factors include changes in cerebral blood flow or central nervous system (CNS) blood pressure that arise from elevated cerebral venous pressure due to mechanical ventilation, barotrauma, apnea, sepsis, or congestive heart failure. Vascular and extravascular factors refer to fragility of vessels in the GM, the site of origin of neuronal and glial cells destined for cortex. In the premature infant, matrix vessels are susceptible to rupture and hemorrhage due to the lack of structural elements that are present in more mature vessels and due to the lack of adequate external tissue support. The severity of GM hemorrhage is typically graded with increasing severity from I to IV. Grade I is subependymal hemorrhage confined to the GM (caudothalamic groove). Grade II denotes intraventricular extension without ventricular dilatation. Grade III is subependymal and intraventricular hemorrhage with HC (Fig. 14). Grade IV refers to

additional hemorrhagic periventricular infarction resulting from subependymal venous occlusion. Poor neurodevelopmental outcome in PT brain injury generally correlates with the higher grades of GM/IVH, parenchymal injury (eg, PVL), and ventriculomegaly.42,43

Posthemorrhagic HC (PHH) occurs in 35% of PT/lowbirth-weight patients with IVH, and approximately 15% of those with PHH eventually require treatment with a VP shunt.42 Acutely, HC occurs due to obstruction of the ventricular system or arachnoid villi by red blood cells and their breakdown products. Arachnoidal reaction, most prominent in the cisterna magna, gradually progresses, resulting in an adhesive arachnoiditis that leads to subacute/chronic HC.6,44 Poor neurodevelopmental outcomes are common in infants with IVH and PHH (especially chronic HC) and include seizures, motor handicaps, cognitive delay, and visual impairment.42 White matter injury (eg, PVL) may be associated with ventriculomegaly due to ex vacuo dilatation. It may be difficult to distinguish ventriculomegaly due to PVL from that associated with PHH, and they may coexist.

FIGURE 6. Hypoxic-ischemic injury in a 25-day-old girl. A, Noncontrast CT at presentation shows diffuse cerebral hemispheric hypodensity with loss of gray-white differentiation and less involvement of the basal ganglia, thalami, and cerebellum (white cerebellum sign). Cerebral swelling is associated with effacement of the sulci and ventricles. Magnetic resonance imaging was done the next day. B, Axial DWI shows diffuse high-intensity, restricted diffusion throughout the cerebral cortex (confirmed by apparent diffusion coefficient map). C, Axial T2 image shows diffusely increased intensity of the cerebral cortex with effacement of fissures and sulci.

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TABLE 3. Causes of HC in Infancy and Childhood

Developmental

Acquired

Chiari II malformation Aqueductal anomalies Congenital cysts/DW malformation Encephalocele Hydranencephaly Craniosynostosis Skull base anomalies Foraminal atresia Immature arachnoid villi Vein of Galen aneurysm

Posthemorrhage Postinfection Posterior fossa tumors Tumors about the third ventricle Cerebral hemispheric tumors

T2/fluid-attenuated inversion recovery (FLAIR) periventricular white matter hyperintensity, loss of the periventricular white matter volume, and irregular ventricular margins are findings characteristic of PVL (Fig. 15).45Y47

Subarachnoid and intraventricular hemorrhage in fullterm infants is less common than in PT infants and may be due to coagulopathy, dehydration, hypoxic-ischemic injury, venous thrombosis, infection, or trauma (including birthrelated trauma) (Fig. 16).48 Although the mechanism of acute and chronic PHH is similar to that for PT infant, outcome is more variable for the term infant.6

Venous hypertension due to venous thrombosis should be considered in infants presenting with unexplained seizures and irritability or in those with hemorrhage or infarct not corresponding to an arterial vascular territory. Venous hypertension may be due to developmental conditions such as

FIGURE 8. Holoprosencephaly in a 5-day-old baby. Axial T1 (A) and midsagittal T2 (B) images show a large monoventricle with large dorsal cyst (white arrow) along with a partial anterior interhemispheric fissure, absent corpus callosum, and small posterior fossa.

malformations of the skull base restricting venous outflow (eg, achondroplasia) or congenital heart disease or pulmonary disease with elevated central venous pressures. It may be acquired due to thrombosis of cerebral veins or sinuses from various causes such as infection, vascular malformation, or coagulopathy (Fig. 17). In an evaluation of suspected venous thrombosis, Doppler US, contrast-enhanced CT, computed tomographic angiography, MRI with MR venogram and gadolinium enhancement, or catheter angiography may be needed to directly demonstrate dural venous sinus thrombosis. Cortical, subependymal, or medullary venous occlusion may not be directly demonstrated by these techniques, although hemorrhages or thromboses may be present in those distributions. The thrombosis may appear as computed tomographic hyperdensity, T1 high-intensity, T2 low-intensity, or GRE hypointensity and can mimic hemorrhage. Intravenous enhancement about the thrombus may be seen as an empty BC[ sign. Depending upon the clinical context, treatment may be directed only to the specific cause (eg, infection) or may

FIGURE 7. Occipital encephalocystocele in 28-week-old fetus. Sagittal T2Ysingle-shot fast spin echo image shows herniation of dysplastic occipital lobe, dura, and CSF through a small occipital defect (white arrow) with associated ventriculomegaly.

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FIGURE 9. Walker-Warburg syndrome in a 2-day-old girl. Axial T2 (A) and sagittal T1 (B) images show marked third and lateral ventriculomegaly with absent septum pellucidum and callosal hypogenesis. There is cortical mantle thinning with an irregular gyral pattern (cobblestone lissencephaly), hypogenesis of the pons and cerebellar vermis (white arrows), and tectal dysplasia with aqueductal stenosis.

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Macrocephaly, ICP, and HC

FIGURE 10. Hydrocephalus due to aqueductal stenosis. A, Sagittal T2 image demonstrates the aqueductal web (white arrow), small fourth ventricle, and marked third ventricular enlargement with ballooning of the anterior and posterior recesses plus downward displacement of the floor into the sella (black arrow). B, Axial FLAIR image shows the markedly dilated lateral ventricles with hyperintense periventricular edema (white arrow).

also include anticoagulation or thrombolysis.49Y53 Increased pressure within the dural sinuses creates a decreased pressure gradient across the arachnoid villi that results in decreased CSF resorption. In the setting of venous hypertension, either HC or pseudotumor cerebri may occur depending on patient age and patency of the cranial sutures. In young infants (less than 18 months) who have open sutures, an expansile calvarium, and soft, undermyelinated, immature white matter, HC is more likely to occur because the ventricles may expand without resistance. Pseudotumor is more common in older infants and children.6

Postinfectious HC Hydrocephalus in infants may be the result of prenatal or

postnatal infection. Prenatal infections occur either by ascending infection from the cervix to the amniotic fluid (usually bacteria or herpes) or via hematogenous dissemination through the placenta (eg, toxoplasmosis, other infections,

FIGURE 12. Communicating HC post-VP shunt in a 4-year-old boy. A, Axial FLAIR image at presentation in early infancy demonstrates marked ventriculomegaly with thinning of the cortical mantle. B, Follow-up axial FLAIR image shows a right parietal ventricular catheter in place (white arrow), small ventricles, and increased mantle thickness. The periventricular white matter hyperintensity (black arrow) likely represents undermyelination vs some leukomalacia or gliosis.

rubella, cytomegalovirus [CMV] infection, and herpes simplex infections and other viruses). During the first 2 trimesters, infection will typically lead to malformations. In the third trimester, destructive lesions occur. Ventriculomegaly is often due to cerebral destruction, but HC may also occur and is most ommon in toxoplasmosis. A comprehensive review of prenatal infections is described elsewhere. Features of the 2 most common entities (CMV and toxoplasmosis) are described below.6,11,53

Congenital CMV infection is a common and serious viral infection among newborns. Depending on the timing of the insult, signs and symptoms include hepatosplenomegaly, microcephaly, chorioretinitis, and seizures.6,53 Affected patients have varying degrees of lissencephaly/polymicrogyria, decreased cerebral white matter, astrogliosis, cerebral calcification, delayed myelination, and cerebellar hypoplasia.54,55 Ventriculomegaly is usually related to cerebral underdevelopment/destruction, rather than HC (Fig. 18). Infection during early gestation tends to result in more severe

FIGURE 11. Cerebral underdevelopment in an infant with tetralogy of Fallot. Axial T2 images (AYC) show lateral ventricular enlargement out of proportion to the third ventricle and temporal horns. The sylvian fissures are wide. Normal hyperintensity is seen within the cerebral white matter due to immaturity. The findings suggest underdevelopment, rather than HC, although a component of communicating HC is always difficult to exclude and may coexist with any cause of underdevelopment or atrophy.

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FIGURE 13. Endoscopic third ventriculostomy. A, Sagittal T1 image shows aqueductal stenosis (white arrow) with third and lateral ventricular enlargement. B, Post-ETV sagittal T2 image shows a CSF flow void across the ETV (black arrow) along with decreased enlargement of the third ventricular and less ballooning of the anterior recesses.

disease. Computed tomography often detects cerebral calcifications (eg, periventricular). Magnetic resonance imaging best demonstrates the parenchymal involvement.11,54,55 Congenital toxoplasmosis may manifest at birth or days to weeks later. There may be generalized or predominantly CNS involvement. Calcifications are common and more random in distribution, including periventricular, cortical, and basal ganglia. Hydrocephalus often results from the granulomatous meningeal or ependymal reaction that can cause aqueductal stenosis and communicating HC. Ventriculomegaly may also occur secondary to cerebral tissue destruction. Malformations of cortical development (eg, polymicrogyria) are uncommon.11,53,56

Postnatal meningitis may be bacterial, viral, fungal, or parasitic and caused by direct (eg, sinus or ear infection) or hematogenous spread. Common etiologies include Gramnegative bacteria (eg, Escherichia coli), group B streptococcus, pneumococcus, Listeria, neisseria, and tuberculosis. In the acute-subacute setting, meningitis can lead to HC due to clumping of purulent fluid along the CSF pathways or due to inflammation of arachnoid granulations with reduced CSF resorption. Chronically, the presence of inflammatory exudate and blood products lead to arachnoiditis. Fungal and granulomatous meningitides are more likely to cause

FIGURE 15. Periventricular leukomalacia. Axial T2 (A) and axial FLAIR (B) images show lateral ventriculomegaly with irregular margins, periventricular high intensities (white arrow), and decreased white matter volume.

clinically significant HC than bacterial and viral infections. The severity of HC is also related to the duration and severity of infection.56Y59 Normal imaging evaluation does not exclude CNS infection. Magnetic resonance imaging may sometimes demonstrate meningeal enhancement. In fungal and granulomatous infection, the meningeal enhancement and thickening often has a predilection for the basal cisterns. Magnetic resonance imaging is mainly used to evaluate the sequelae and complications of meningitis. Arachnoid loculations due to arachnoid scarring may occur and simulate arachnoid cysts (ACs). Ventricular dilatation may be shown by MR or CT. Other complications of meningitis, including venous thrombosis, infarction (arterial or venous), ventriculitis, cerebritis, abscess, and subdural empyema, are best delineated with MRI (Fig. 19).56,58,59

DEVELOPMENTAL MALFORMATIONS

Chiari II Chiari II malformation accounts for about one third

of infantile HC. Almost all present at birth with a MMC.

FIGURE 14. Preterm PHH. Sagittal T1 (A) and axial T2 (B) images shows moderate-to-marked ventriculomegaly with left subependymal and intraventricular hemorrhages (arrows) that are T1 hyperintense and T2 hypointense. Midsagittal T2 image (C) shows a widely patent aqueduct (black arrow) and large cisterna magna, consistent with communicating HC.

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